None constant aperture zoom lenses

I was just wondering if your using for example a f3.5-f4.5 none constant aperture zoom at what point does it change aperture during the zoom travel ? because this could be very significant if it is used with a hand held light meter or none TTL strobe, what I'm asking is, is the transition gradual throughout the zoom range ?.

benjiboy said:

I was just wondering if your using for example a f3.5-f4.5 none constant aperture zoom at what point does it change aperture during the zoom travel ? because this could be very significant if it is used with a hand held light meter or none TTL strobe, what I'm asking is, is the transition gradual throughout the zoom range ?.

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It is gradual on the lenses I have.

On lenses of this type I tend to avoid using the wider/variable apertures. With the example you gave I'd treat it as a 4.5 and use A or M to keep it out of the variable range.

It's gradual on all variable aperture zooms.

Basically (though not exactly), the focal length changes while the pupil size remains the same. And given that the f-number is f(ocal length)/pupil size, any change in focal length, no matter how minute, will change the f-number.

And that provides the clue to how to use such a lens with non-TTL metering: first calculate the pupil size by doing the sum focal length/f-number at that focal length (always given for both extremes, so doable), and jot that down somewhere handy.
Then after changing focal length do the sum backwards again: f-number = focal length set/number calculated in first step.

What you also could do is do the sums and calculate at what focal length setting you would need to add 1/3 or 1/5 stop, and mark those points on the focal length scale of the lens.

Mind you: while close enough, it's not necessarily correct, nor exact. It depends on what happens inside that lens, and that varies with design.
But it works.

And another mind you: this changes with focus setting too. The closer you get, the more light you lose.
And that can be a significant amount not just when doing macro. Especially not when using wildly asymmeric lenses, which zooms are.

Thanks guys , I suspected that was the case, I just have one variable aperture zoom a 35-70 my others are constant aperture.